Why do I lose my selection when switching to the 'Paint' mode of the UV Editor?
The Face Selection Masking in the the 3d view header is only available in the 3D View in Texture paint -  the UV Image Editor in Paint mode does not respect this restriction. If you use the UV Image Editor for the face selection, and then tab to Texture paint mode in 3d view you can then turn on Face Selection masking and see the focused selection ready to paint - and it will only deliver paint in that face plus whatever bleed amount you have set in the Options tab in the toolbar.

For now, the best option is to use the fill brush in the 3d view with the face selection masking. Because of this, I added the vertex group selectionint the Texture Paint Plus addon version Spirou4d and I have been changing. Texture Paint Plus CDMJ fork I have a video on it as well.youtube video on selection masking

I haven't found a way to be in edit mode and in texture paint mode yet, so for now the keeping the selections in sync won't work while painting.